Photodynamic therapy (PDT) is a two step process. First, a photoactivatable drug or agent (interchangeably referred to herein as a “photosensitizer” or “PS”) is administered either systemically or locally. Then after an appropriate interval, during which time the photoactivatable drug accumulates in target tissue more than in non-target tissue, irradiation with electromagnetic irradiation, such as visible (non thermal) light, is applied to a treatment area for a short period of time. During the time of irradiation the drug acts as a catalytic photosensitizer, absorbing a photon and transferring this energy to oxygen. This process converts the available stable, benign molecular oxygen into cytotoxic singlet oxygen or other toxic oxygen species, which kills targeted cells or otherwise alters or modulates cellular activity in them. PDT is widely used in the treatment of cancer, and in certain ophthalmic conditions characterized by neovascularization in the eye, for example, in age related macular degeneration (AMD). Visudyne® (Novartis Ophthalmics, Inc.), containing a green porphyrin photosensitizer, verteporfin, is approved in many countries for the treatment of choroidal neovascularization and AMD.
While many photosensitizers are hydrophobic or otherwise water insoluble, they are often needed in water based or otherwise aqueous environments in which target cells are found. As such, multiple systems have been developed as delivery vehicles for such agents. Preferably, pharmaceutically acceptable delivery vehicles for photosensitizers are manufactured simply and cost effectively, while maintaining the properties of efficient drug delivery.
Nanoparticles are solid colloidal particles formed by the association of suitable polymers. Nanoparticles are produced by mechanical or chemical means and can be formulated to contain an active substance such as a hydrophobic drug in association with the polymers. Nanoparticles thus provide an alternative to other colloidal carriers such as liposomes, micelles, niosomes (non-ionic surfactant vesicles), and microemulsions.
Synthetic polymers used in the preparation of nanoparticles include polyesters such as poly (lactic acid), poly (lactide-co-glycolide) and poly (ε-caprolactone) (U.S. Pat. No. 4,968,350). Nanoparticles are usually prepared using one of the following methods: emulsion evaporation, solvent displacement, emulsification-diffusion and the salting-out technique. These techniques are similar in that they involve an organic solution containing the nanoparticle components and an aqueous component containing stabilizers that act as the dispersion medium for the nanoparticles.
Biodegradable nanoparticles based on polyester polymers such as poly(D,L-lactide-co-glycolide) (PLGA) and poly(D,L-lactide) (PLA) have been widely investigated as parenteral delivery systems (Allémann et al., 1998; Kawashima et al., 1998; Rodrigues Jr et al., 1995; Scholes et al., 1993; Smith et al., 1986). Polyester polymers, approved by the Food and Drug Administration, are of interest due to their physico-chemical and biological properties (Vert 1987; Vert et al., 1998). Nanoparticles have been used to modify the pharmacokinetics of drugs, as detailed in the following: Roland, Clinical pharmacokinetics of doxorubicin in hepatoma patients after a single intravenous injection of free or nanoparticles bound anthracycline, Int.J.Pharm., 54:113(1989); J. Kattan, J. P. Droz, P. Couvreur, J. P. Marino, A. B. Laroze, P. Rougier, P. Brault, H. Vranckx, J. M. Grognet, X. Morge and H. Sancho-Carnier, Phase I clinical trial and pharmacokinetic evaluation of doxorubicin carried by polyisohexylcyanoacrylate nanoparticles, Invest. New Drugs, 10:191 (1992); H. S. Yoo, J. E. Oh, T. G. Park, Biodegradable nanoparticles containing doxorubicin-PLGA conjugate for sustained release, Pharm. Res., 16:1 114 (1999); E. Allémann, R. Gurny, E. Doelker, F. S. Skinner, H. Schutz, Distrubution, kinetics and elimination of radioactivity after intravenous and intramuscular injection of 14C-savoxepine loaded poly(D,L-lactic acid) nanospheres to rats, J. Controlled Release 29: 97-104 (1994); R. Jalil, Biodegradable poly(lactic acid) and poly(lactide-co-glycolide) polymers in sustained drug delivery, Drug Dev. Ind. Pharm., 16:2353 (1990), C. Tasset; N. Barette; S. Thysman; J. M. Ketelslegers; V. Preat et al , Polyisobutylcyanoacrylate nanoparticles as sustained release system for calcitonin, J. Control. Rel., 33: 23 (1995); T. Harmia; P. Speiser, J. Kreuter, Nanoparticles as drug carriers in ophthalmology, Pharm. Acta. Helv., 62: 322(1987); and M. El-Samaligy; P. Rohdewald, Triamcinolone diacetate nanoparticles, sustained release drug delivery system suitable for parenteral administration Pharm. Acta. Helv., 57(7): 201-4 (1982).
Traditionally, biodegradable nanoparticles have served as depots of entrapped drug which are slowly released from the polymer matrix. This has been thought to be an effect of a slow polymer degradation to result in a slow release of the drug over time. For example, intramuscular injections of long-acting injectable microcapsules composed of poly(DL-lactide)-co-glycolide containing contraceptives into women provided controlled release (Beck LR et al. A. J. Obstet. Gynecol. 147, 815-821, 1983). Also, doxorubicin nanoparticles were thought to deposit in an intrahepatic reservoir and slowly release when administered in polyisohexylcyanoacrylate and polymethacrylate nanoparticles (Kattan J. et al. Investigational New Drugs, 10, 191-199, 1992, Rolland A International Journal of Pharmaceutics, 54, 113-121, 1989).
For parenteral administration, nanoparticulate formulations must meet the pharmacopoeial requirements of sterility. However, sterilization of such polymeric devices by a satisfactory technique remains a challenge. The chemical or physical lability of the polymer matrix usually limits most conventional methods for obtaining acceptable sterile products. For example, sterilization by autoclaving can induce degradation of polyesters and drug by hydrolysis. These polymers are also heat sensitive due to their thermoplastic nature (Athanasiou et al., 1996). Chemical sterilization by gases such as ethylene oxide may result in toxicity problems to toxic residues. Numerous studies have shown the effects of γ-irradiation on the stability and the safety of colloidal carriers based on polyesters, principally microparticles (Hausberger et al., 1995; Mohr et al., 1999; Montanari et al., 1998; Volland et al., 1994). Gamma-irradiation was shown to affect drug loaded polyester microparticle properties in several ways such as radiolytic reactions, chain scission and cross-linking (Volland et al., 1994). These reactions may have consequences on the nominal drug content, the drug release pattern and the bioresorption of the system. Moreover, the encapsulated drug may degrade upon exposure to gamma irradiation. Therefore, the selection of a suitable sterilization method for such type of formulations is crucial to ensure their physical and chemical integrity, their performance and safety in vivo.
As an alternative technique, sterile filtration through 0.22 μm membrane filters has been used for chemically or thermally sensitive materials since it has no adverse effect on the polymer and the drug. The presence of particles with sizes above the sterilization membrane cutoff, however, tends to result in membrane clogging and decreased efficiency of filtration. It would be advantageous to have methods for the preparation of nanoparticles with a mean size significantly below the sterilization membrane cut-off and with a narrow size distribution to avoid membrane clogging. The ability to produce nanoparticles in the size range of 100-150 nm would be attractive for targeting sites located outside the vascular system. Some tumours, which possess a defective microvasculature, exhibit an increased vascular permeability favouring the accessibility of colloidal carriers to extravascular tumoral cells (Douglas et al., 1987).
Citation of documents herein is not intended as an admission that any is pertinent prior art. All statements as to the date or representation as to the contents of these documents is based on the information available to the applicant and does not constitute any admission as to the correctness of the dates or contents of these documents.